1. Field of the Invention
The present invention relates to a slip control system for a vehicle and, particularly, to a slip control system for a vehicle with a mechanical supercharger of which supercharging pressure is controlled by a supercharging pressure control valve.
2. Description of Related Art
Where a slip of a drive wheel of the vehicle takes place when the vehicle starts running and when the vehicle is in an accelerating condition, a grip force between a tire and road surface is reduced so as to cause various problems such as running instability, deterioration of fuel consumption efficiency and the like. In a vehicle with a supercharger, the engine output torque is enhanced by the supercharger so that the slip condition of the drive wheel tends to cause a reduction in traction efficiency of the driving force.
Conventionally, there have been proposed various slip control systems such as that disclosed in Japanese Patent Public Disclosure No. 2-61339, laid open to the public in 1990. According to this Japanese publication, a change of the engine output torque is calculated based on the engine speed and the throttle valve opening to control the amount of the restriction of the engine output torque in accordance with the slip amount of the drive wheel. There are provided control value calculation means for calculating a control value of the throttle opening, based on the amount of the change of the engine output torque and the slip amount of the drive wheel, and switch means for switching calculating characteristics of the control value, by the control value calculation means, in accordance with operation of the supercharger. This is done to match the control value of the throttle opening with the engine output torque which is changed corresponding to the operation of the supercharger. As a result, even where the engine output characteristics are changed in accordance with the operating condition of the supercharger, the engine output torque can be controlled with good response to the slip of the drive wheel so as to cause an actual rotation speed of the drive wheel to converge with a target value thereof quickly.
In the above slip control device disclosed in the Japanese publication, the restriction control of the engine output torque is made by reducing the opening of the sub-throttle valve disposed in the vicinity of and in series with a main throttle valve.
Other than the above type of slip control system, in which the engine output torque is controlled by the throttle valve opening, there has also been known a slip control system in which the engine control is made through an ignition timing control and/or fuel injection control (a fuel cut) to restrict the engine output torque and thereby reduce the slip amount of the drive wheel. In this type of slip control, a retard value of the ignition timing is determined in view of a control level of the slip value. Alternatively, fuel injection for a specific cylinder is cut in view of the slip control level to reduce the engine output.
It has been proposed to apply the above type of slip control system to a vehicle having an engine with the supercharger so as to restrict the engine output torque.
It should be noted, however, that when the ignition timing is retarded for reducing the engine output in the supercharging condition of the engine, so called delayed combustion takes place due to the large amount of the intake air introduced into the combustion chamber. Delayed combustion undesirably causes an increase in the temperature of the catalyst. Likewise, when the fuel cut is made to restrict the amount of fuel injection, the temperature of the catalyst is also increased so as to damage the catalyst. Further, when the supercharger is effected, or in a supercharging condition, the engine output is abruptly increased so that an initial spin of the wheel or first slip of the wheel, in which a slip amount exceeds a certain level, where the traction control is desired, for the first time during vehicle running, is large as compared with that under a condition where the supercharger is out of operation, or in a non-supercharging condition. Under such a supercharging condition, it is necessary to reduce the slip value as soon as possible.